Currently in the access network field, optical access booms when a Digital Subscriber Line (DSL) is fully developed, and particularly, a point-to-multipoint optical access technology, that is, a Passive Optical Network (PON), attracts attention again. Compared with point-to-point optical access, a PON central office is capable of dividing one fiber into decades of or even more paths of fibers to be connected to users, thereby significantly reducing a network-establishment cost. Currently, representative PON technologies include a Gigabit-Capable Passive Optical Network (GPON) technology and an Ethernet Passive Optical Network (EPON) technology, in which the GPON technology has properties of having a high line rate and complete maintenance and management functions.
As shown in FIG. 1, a structure of a PON system includes three parts, that is, an Optical Line Termination (OLT), an Optical Distribution Network (ODN), and an Optical Network Unit (ONU)/Optical Network Termination (ONT).
The OLT provides a network side interface for the PON system, and is connected to one or more ODNs. The ODN is a passive optical splitter, transmits downlink data of the OLT to each ONU in a drop manner, and simultaneously transmits uplink data of multiple ONUs/ONTs to the OLT in an aggregating manner. The ONU provides a user side interface for the PON system, and an uplink is connected to the ODN. If the ONU directly provides a user port function, such as an Ethernet user port used by a Personal Computer (PC) for accessing the Internet, the ONU is referred to as an ONT, and the ONU mentioned in the following collectively includes the ONU and the ONT.
The ODN is generally divided into three parts, that is, a passive optical splitter, a feed fiber, and a distribution fiber and a drop fiber, in which the distribution fiber and the drop fiber may be collectively referred to as a branch fiber. FIG. 1 is a structural diagram of an ODN with level-2 splitting, and an ODN with only level-1 splitting only has the feed fiber and the drop fiber.
A link from the OLT to the ONU is referred to as the downlink, and a link from the ONU to the OLT is referred to as the uplink. Due to optical characteristics, the downlink data is broadcasted to each ONU, and the OLT allocates sending intervals for the uplink data sending of each ONU, in which time division multiplexing is performed.
The uplink of the PON system adopts a wavelength of 1310 nm, and the downlink of the PON system adopts a wavelength of 1490 nm. Light of the uplink and the downlink may be transmitted in the same fiber or by adopting different fibers.
The line between the OLT and the ONU may be abnormal, such as breaks, so that normal services are influenced. Therefore, the line needs to be detected, so as to evaluate a line state.
The line may be detected in the following manners. In the first manner, the OLT correspondingly modulates or encodes a downlink data signal, so as to superpose test signals. After receiving a backward reflected signal and/or scattered signal of a downlink signal, a receiver of an Optical Time Domain Reflectometer (OTDR) obtains a reflected and scattered signal of the test signal superposed on the downlink data signal after removing a reflected and scattered signal of a downlink digital signal through filtering. When being transmitted through a fiber, the test signal generates a backward scattered signal due to a property (medium non-uniformity) of the fiber itself, or generates a reflected signal due to a fiber link event (connection, breakage, and fiber end). Line attenuation situations distributed along a fiber length and an event curve on the line are calculated and obtained according to an intensity of the detected backward scattered signal or reflected signal and time of arrival, so as to implement the detection of the ODN. This method needs to perform amplitude modulation control on a laser in a downlink direction, and to modulate an amplitude of a data signal by 5% to 10%, and the backward reflected and scattered signals are very weak, so that the processing difficulty is large.
A test method shown in FIG. 2 may also be adopted, in which a laser of a data transceiver sends a downlink data signal with a wavelength of λd, and a receiver of the data transceiver receives an uplink data signal with a wavelength of λu. A laser of the OTDR sends a test signal with a wavelength of λt, and a receiver of the OTDR still receives a backward scattered signal of the test signal with the wavelength of λt and processes the received backward scattered signal, so as to implement the monitoring of the ODN link. Because a test wavelength is different from a data wavelength, a filter needs to be placed before the ONU to get rid of the influence of the test wavelength on the receiver of the ONU, so that the cost is high.